The need for miniaturized power sources for micro-electro-mechanical systems (MEMS) and micro-electronics has long been recognized. Much work has already been done on micro-scale batteries, and micro-scale heat engines. Micro-scale heat engines are a particularly attractive option, because of the very high density energy storage afforded by the hydrocarbon fuels they burn. Thus, a micro-heat engine which could convert the chemical energy stored in a hydrocarbon fuel to mechanical or electrical energy could form the basis of a very compact power supply.
Piezoelectric thin films have been used for years as power transducers in MEMS and micro-electronic devices. Piezoelectric films are an attractive option for power transduction because of the relative ease with which such devices can be produced using conventional micro-machining methods. Generally speaking, micro-machining involves processing techniques, such as microlithography and etching, that were developed and refined for use in the manufacture of integrated circuits. Micro-machining allows fine control of dimensions and is commonly employed for producing parts from silicon. However, micro-machining is not restricted in its application to the formation of workpieces from silicon or other materials conventionally used in the manufacture of integrated circuits, and it is known to apply micro-machining to other materials.
In most applications of piezoelectric films, such as in micro-actuators, pumps, and valves, electrical power is converted to mechanical power. Micro-sensors that utilize piezoelectric films also have been used for mechanical-to-electrical transduction, however, such devices are not capable of producing usable electrical power to any significant degree. Thus, it would be desirable to utilize piezoelectric thin films for converting energy in one form, such as thermal energy or kinetic energy, to useful electrical energy to power MEMS and micro-electronic devices.
Along with the need for miniaturized power sources is the need for micro-devices that are designed to remove heat from MEMS and micro-electronics. In particular, integrated-circuit manufacturers are already reaching limits on micro-processor speed and performance imposed by high operating temperatures. Consequently, reducing the operating temperatures of chips by removing waste heat through active cooling is considered to be among the most promising strategies available to the microprocessor industry for overcoming these obstacles. Thus, it would be desirable to implement a piezoelectric film in a micro-heat pump for cooling applications of MEMS and micro-electronics.
Many MEMS devices have been developed that rely on thermal energy for actuation. This energy can be supplied in a variety of ways. For example, there are micro-systems that receive heat from electrical resistance heaters, external sources, and chemical reactions. The ability to control the heat transfer into and out of these MEMS devices is essential to their performance. The necessity for precise thermal management is especially critical for micro-devices that operate at high frequencies, such as micro-thermopneumatic pumps, bi-layer electrical relays, and micro-heat engines. Often, it is the inability to rapidly reject heat that limits the operating frequencies of such devices. Thus, there is a strong need for a thermal switch that enables the precise control of heat transfer into and out of such MEMS devices.